There presently are over 500,000 wooden vehicular bridges in the United States assembled from chemically treated lumber. An estimated forty percent of them are in need of repair or replacement.
There are several types of chemically treated lumber such as creosoted lumber and pressure treated lumber. These materials are relatively inexpensive to make and use, and they are just as versatile as any other form of wood. They also have enhanced resistance to microbial and to fungal degradation and to water.
However, the increasing popularity of chemically treated lumber has some negative repercussions that are just now being realized. Chemically treating lumber takes a perfectly useable, recyclable, renewable resource and renders it toxic. For example “pressure treated” or “CCA” lumber is treated with very poisonous chromated copper arsenic and cannot be burned. While CCA lumber can be buried, the leaching of toxic chemicals makes such disposal strategies undesirable. The disposal of creosoted lumber requires the use of special incinerators. These materials are becoming far more difficult and expensive to dispose of than to use. However, because of the long useful life of these materials, the economic and environmental impact of chemically treated lumber is just beginning to be felt.
Structural recycled plastic lumber represents a possible alternative to chemically treated lumber. U.S. Pat. Nos. 6,191,228, 5,951,940, 5,916,932, 5,789,477, and 5,298,214 disclose structural recycled plastic lumber composites made from post-consumer and post-industrial plastics, in which polyolefins are blended with polystyrene or a thermoplastic coated fiber material such as fiberglass. These structural composites presently enjoy commercial success as replacements for creosoted railroad ties and other rectangular cross-sectioned materials. The market has otherwise been limited for structural recycled plastic lumber, because it is significantly more expensive than treated wooden beams on an installed cost basis, despite the use of recycled waste plastics.
This significant cost difference became more evident in the construction of bridge structures in which pressure-treated wooden beams were replaced with structural recycled plastic lumber composite beams. While as strong as CCA treated wood, the recycled plastic composite beams were not as stiff, and tended to sag, or “creep.” It was possible to compensate for this by increasing beam dimensions and using more beams of rectangular cross-section. However, this just added to the already increased cost for materials and construction in comparison to treated lumber.
Structural beams that do not “creep” can also be prepared from engineering resins such as polycarbonates or ABS. However, these are even more costly than the structural composites made from recycled plastics. There remains a need for structural materials based on recycled plastics that are more cost-competitive with treated lumber on an installed cost basis.